Genetics Continued
• Test Cross
•Codominance
•Incomplete Dominance
•Sex Determination
•Sex-Linked
•Dihybrid Cross
Test Cross
• Remember: The dominant phenotype has 2
possible genotypes.
– Example:
TT
OR
Tt
• When trying to find out the genotype of a
dominant phenotype ALWAYS cross it with
the homozygous recessive.
• Then look at the children because they will
tell you if the parent is homozygous
dominant or heterozygous.
Example of a Test Cross Problem
• In Sharks, sharp teeth are dominant (T) to
dull teeth (t). If I found a sharp toothed
shark, how can I find out its genotype?
Prove it with Punnet Squares.
Answer: Cross it with a
homozygous recessive (tt), then
look at the children.
T
T
t
Tt
Tt
t
Tt
Tt
All Children Sharp Teeth
T
t
t
Tt
tt
t
Tt
tt
50% Sharp 50% Dull
Codominance
• Codominance means both phenotypes are
dominant.
– Example: Orange goldfish (OO) are
codominant with white goldfish (WW). A
heterozygous goldfish (OW) would be both
Orange and White (spotted).
Incomplete Dominance
• Incomplete dominance means that neither
phenotype is dominant over the other.
• It is a blending of the phenotypes.
– Example: Red octopus (RR) are incompletely
dominant to Blue octopus (R’R’). A
heterozygous (RR’) would be purple (Red +
Blue = Purple)
Sex Determination
• All animals have 1 pair of chromosomes
that determine their sex.
• In MOST animals, females are XX and
males are XY.
• Therefore, when crossing a male (XY) with
a female (XX), what is the chance of having
a boy? A girl? Who passes the chromosome
to determine sex, Mom or Dad?
Answer:
DAD
X
X
X
XX
XX
Y
XY
XY
MOM
• 50% Chance for a boy
• 50% Chance for a girl
• Dad determines the sex (gives X or Y). Mom can
only give an X.
Sex Linked
• There are some traits that are located on the
sex chromosomes (X or Y), which are
called sex linked.
• These traits are often common in men
because they only have one X chromosome,
but women have two X chromosomes to
protect them.
Example Sex Linked Problem
• Albinism (lack of skin color) is a recessive sex
linked trait carried on the X chromosome (Xa). If a
dolphin is not an albino they have a dominant X
chromosome (XA).
• A heterozygous female dolphin is crossed with a
normal male dolphin. What is the chance the girl
dolphins will be albino? What is the chance the
boy dolphins will be albino?
Answer:
DAD
XA
Xa
XA
XAXA
XAXa
Y
XAY
XaY
•0% chance that the girls will be an albino
•50% chance of the boys will be an albino
MOM
Dihybrid Cross
•
•
A dihybrid cross is a 16 box Punnet Square
It is used to cross 2 different traits at the same
time
–
Ex: Height and Color
1) Write the genotypes of the parents
2) Write the possible gametes of each parent
Remember: 1st-1st, 1st-2nd, 2nd-1st, 2nd-2nd
3) Write the gametes outside the Punnet Square
4) Then fill in the boxes like a Punnet Square
Remember:
1st-1st, 1st-2nd, 2nd-1st, 2nd-2nd
• Practice: TtFf
T t F f
TF
Tf
tF
tf
TtFf x TtFf
T=sharp, t=dull
F=fancy, f=plain
TF
Tf
tF
tf
TF
TTFF
TTFf
TtFF
TtFf
Tf
TTFf
TTff
TtFf
Ttff
tF
TtFF
TtFf
ttFF
ttFf
tf
TtFf
Ttff
ttFf
ttff